In recent years, for absorption of large quantities of water, a water-absorbent agent such as water-absorbent resin has been widely adopted as one of the materials constituting sanitary articles such as paper diapers, sanitary napkins, and adult incontinence pads. Also, for absorbing and holding of water, such a water-absorbent agent (water-absorbent resin, etc.) has also been widely adopted as soil conditioners and drip sheets for food.
Examples of a conventionally known water-absorbent agent are as follows: (1) a partially neutralized and crosslinked polyacrylic acid, (2) hydrolyzed copolymer of starch-acrylonitrile, (3) neutralized graft polymer of starch-acrylic acid, (4) saponified copolymer of vinyl acetate-acrylic ester, (5) a crosslinked polymer of a hydrolyzed copolymer of acrylonitrile or a crosslinked polymer of a hydrolyzed copolymer of acrylamide, and (6) a crosslinked cation monomer, etc.
The properties which the water-absorbent resin is desired to possess, in order to be adopted as sanitary articles, include, for example, (a) high absorbency and high absorbing rate to be manifested upon contact with aqueous liquids such as body fluids, (b) liquid permeability, (c) high strength exhibited by a gel swollen with liquid, and (d) an ability to aspirate water from a substrate impregnated with aqueous liquid.
However, these properties are not necessarily proportionally related to one another such that, for example, as absorbency of water-absorbent resin increases, such properties as the liquid permeability, the gel strength, and the absorbing rate decrease. In order to improve a balance of the various water-absorbent properties of the water-absorbent resin, various techniques of crosslinking the surface region of the water-absorbent resin have been suggested.
As such techniques, the following methods in which specific compounds are adopted as a crosslinking agent are known. (a) A method using a polyhydric alcohol as a crosslinking agent (Japanese Unexamined Patent Application No. 180233/1983 (Tokukaisho 58-180233), and Japanese Unexamined Patent Application No. 16903/1986 (Tokukaisho 61-16903)), (b) a method using a polyglycidyl compound, a polyaziridine compound, a polyamine compound, and a polyisocyanate compound as a crosslinking agent (Japanese Unexamined Patent Application No. 189103/1984 (Tokukaisho 59-189103)), (c) a method using a glyoxal as a crosslinking agent (Japanese Unexamined Patent Application No. 117393/1977 (Tokukaisho 52-117393)), (d) a method using a polyvalent metal as a crosslinking agent (Japanese Unexamined Patent Application No. 136588/1976 (Tokukaisho 51-136588), Japanese Unexamined Patent Application No. 257235/1986 (Tokukaisho 61-257235) and Japanese Unexamined Patent Application No. 7745/1987 (Tokukaisho 62-7745)), (e) a method using a silane coupling agent as a crosslinking agent (Japanese Unexamined Patent Application No. 211305/1986 (Tokukaisho 61-211305), Japanese Unexamined Patent Application No. 252212/1986 (Tokukaisho 61-252212) and Japanese Unexamined Patent Application No. 264006/1986 (Tokukaisho 61-264006)), (f) a method using an epoxy compound and a hydroxy compound as crosslinking agents (Japanese Unexamined Patent Application No. 132103/1990 (Tokukaihei 2-132103)), and (g) a method using an alkylene carbonate as a crosslinking agent (DE-4020780, and U.S. Pat. No. 5,409,771).
Also, as a technique of uniformly carrying out surface crosslinkage in which a crosslinking agent is uniformly spread over the surface of the water-absorbent resin during a crosslinkage reaction, for example, the following methods, applied when adding the crosslinking agent, are known. (h) A method using an inorganic inactive powder (Japanese Unexamined Patent Application No. 163956/1985 (Tokukaisho 60-163956) and Japanese Unexamined Patent Application No. 255814/1985 (Tokukaisho 60-255814)), (i) a method using a dihydric alcohol (Japanese Unexamined Patent Application No. 292004/1989 (Tokukaihei 1-292004)), (j) a method using an ether compound (Japanese Unexamined Patent Application No. 153903/1990 (Tokukaihei 2-153903)), and (k) a method using alkylene oxide additive of a monchydric alcohol, an organic acid salt, lactam, etc. (Japanese Unexamined Patent Application No. 200046/1994 (Tokukaihei 6-200046), and EP-0555692).
With the described surface processing methods, some improvements in the balance of the various water-absorbent properties of the water-absorbent resin are attained, yet further improvements are needed to reach a desirable level. That is to say, in light of recent trend of an absorbent of thin sanitary articles containing a large amount of water-absorbent resin, considering the properties required for such water-absorbent resin, the water-absorbent resin made by the described methods has not reached a satisfactory level. Therefore, a further improvement in the quality of the water-absorbent resin is demanded.
The properties required for water-absorbent resin contained in high concentration in the absorbent are (1) high absorbency under no applied pressure and (2) water-absorbing properties, such as absorbency and liquid diffusivity under heavy load and high pressure, which are superior than that of conventional water-absorbent resin. Also, in the case where the absorbent containing in high concentration the water-absorbent resin is used for an extended period of time, a water soluble component (mainly water soluble polymer component) of the water-absorbent resin is gradually removed. This may result in lowering of diffusivity of aqueous liquid such as body fluids, and an increase in the amount of backlash of the aqueous liquid. For this reason, it is demanded to further reduce the amount of the water soluble component in the water-absorbent resin.
Incidentally, as a common manufacturing method of the water-absorbent resin, the following methods are known. (I) A method in which a monomer of acrylic acid or other compounds as a main component, which have been neutralized, are polymerized, and (II) a method (so-called a post-neutralization polymerization method) in which after polymerizing a monomer of acrylic acid or other compounds as a main component, which have not been neutralized or have been neutralized at a relatively low neutralization ratio within a predetermined range, resulting hydrogel polymer is neutralized as required. For example, the method (I) is adopted as the manufacturing method of water-absorbent resin disclosed in Japanese Unexamined Patent Application No. 126310/1989 (Tokukaihei 1-126310), and U.S. Pat. No. 4,985,518. However, in method (I), because a relatively large amount of water soluble component is present, when the water-absorbent agent is adopted in sanitary articles, there is a chance that the diffusivity of aqueous liquid such as body fluids is lowered and the amount of backlash of the aqueous liquid is increased. For this reason, the method (II) is considered to be more desirable as a manufacturing method of water-absorbent resin, and is suitably adopted as a method of reducing the water soluble component while maintaining high absorbency of the water-absorbent resin under no applied pressure.
Specifically, the following methods are known as an example of method (II). (1) A method in which after polymerizing the acrylic acid in the presence of a vinyl crosslinking agent, the acrylic acid thus polymerized is neutralized with alkali metals, and resulting water containing neutralized gel is further crosslinked by divalent metal ions (U.S. Pat. No. 4,295,987), (m) a method in which an alkali metal containing compound is added to a hydrogel polymer which has been prepared by polymerizing monomers containing a free acid group such as carboxylic acid, and at least 50 mole percent of the acid group of the hydrogel polymer are neutralized (U.S. Pat. No. 4,654,039), (n) a method in which an alkali metal containing compound is added to a hydrogel polymer which has been prepared by polymerizing a monomer containing a free acid group such as carboxylic acid using a copolymerizable crosslinking agent, and 50 mole percent to 90 mole percent of the acid group of the hydrogel polymer are neutralized, (o) a method in which an alkali metal containing compound is added to a hydrogel polymer which has been prepared by polymerizing a monomer containing an acid group such as carboxylic acid, and after neutralizing 50 mole percent to 90 mole percent of the acid group of the hydrogel polymer, the hydrogel polymer is crosslinked to a compound having at least two or more reactive groups which can undergo reaction with the acid group of the hydrogel polymer, and/or an alkali metal base of the acid group (Japanese Unexamined Patent Application No. 103606/1989 (Tokukaihei 1-103606), and Japanese Unexamined Patent Application No. 103615/1989 (Tokukaihei 1-103615), (p) a method in which a monomer component, which is a 10 mole percent to 50 mole percent neutralized acid group containing monomer, is polymerized so as to carry out neutralization further (Japanese Unexamined Patent Application No. 144404/1989 (Tokukaihei 1-144404), and U.S. Pat. No. 4,985,514), and (q) a method in which after adiabatically polymerizing, using a specific amount of 3 types of specific polymerization initializing agents, acrylic acid until the amount of residual monomers is not more than 1000 ppm, resulting polyacrylic acid is neutralized at a neutralization ratio of 50 mole percent to 100 mole percent, thereafter the hydrogel polymer is crosslinked by adding a specific amount of a crosslinking agent after neutralization, and the crosslinked hydrogel polymer is dried and pulverized (Japanese Unexamined Patent Application No. 174414/1991 (Tokukaihei 3-174414), and U.S. Pat. No. 5,145,906).
However, as a result of studying each of the described methods, the inventors of the present invention found that in the water-absorbent resin as obtained by neutralization after polymerizing acid group containing monomers as in method (II), while it was possible to reduce the water soluble component, the effect of crosslinkage was not sufficient even when the surface crosslinkage process was carried out.
Namely, specifically, in method (II), it was found that while it was possible to improve to a certain level the absorbency under applied pressure when the applied load is low (for example, 20 g/cm.sup.2), it was difficult to improve absorbency under high pressure (for example, 50 g/cm.sup.2). Also, even when a water-absorbent agent with high absorbency under high pressure was obtained, it was difficult to stably obtain such a water-absorbent agent. Indeed, in any of the described methods in accordance with method (II), it was difficult to stably obtain a water-absorbent agent with high absorbency under high pressure.
For this reason, there is a need for (i) a water-absorbent agent having high absorbency not only under no applied pressure but also under high pressure in which the amount of water soluble component is reduced compared with a conventional water-absorbent agent, which can be suitably adopted as a thin absorbent, as well as (ii) a method of manufacturing such a water-absorbent agent.
Also, the inventors of the present invention found after studying that, as described above, in the water-absorbent resin obtained by neutralization after polymerizing the acid group containing monomers, when the water-absorbent resin takes a form of swollen gel by absorbing an aqueous liquid, the pH in the swollen gel is changed with time. Thus, when such a water-absorbent resin is adopted in sanitary articles, there arises a problem of safety due to residual alkali used in neutralization and a problem of unstable pH in the sanitary articles. Therefore, as a water-absorbent agent which can be suitably adopted as an absorbent of sanitary articles, etc., there is a need for a water-absorbent agent whose change in pH with time is small when the water-absorbent agent takes a form of swollen gel by absorbing aqueous liquid.